The ERJs used by SAs regional carriers are the same as an ERJ in the US if you can find anyone still using the ERJ-135, but they are exactly the same as the ERJ-145 in terms of seat size. I do seem to recall that they had the valet system for larger carry on items.

I've only flown SA in economy once, and I honestly don't remember if I checked my regular rollaboard or did carry on. But I'm one of those who often does the rollaboard plus laptop bag on top of it. On the mainline connection however I didn't particularly notice any aggressive checking of carry on bag sizes by gate agents but again here my experience is limited as it was boarding one of the long haul A340 flights which they may be less of a sticker about vs a domestic A320 and we got to the gate well after boarding had started and went straight to the Business Class queue (*G benefit).